Compare 2 images in PHP

Question

Comparing two images to see if they are both the same files is easy: throw the files MD5. But is it possible or even plausible to determine if two images are same by using PHP GD to get the difference of the two images? If we were to get the difference of the two, and it was all white (I'd assume white or even black), then we would now know whether they were both the same photo?

Side note: I'd like to know if it's possible to get two images of equal size to create an onion skin effect: 50% transparency on one and 50% on the other.

Do you still need the transparency effect? I answered the title question, but could code the side note too. — Leopoldo Sanczyk, Sep 04 '20 at 06:40

kander · Accepted Answer · 2020-07-28T10:08:32.820

22

Most of the other answers refer to using various hashing functions. The question explicitly is asking about comparing the contents of the images, not about comparing the files.

This means you end up having to actually understand the contents of the image. In PHP there are two extensions often used for this, ImageMagick and GD.

ImageMagick offers various tools you can use for this, through the PHP ImageMagick extension.

http://www.php.net/manual/en/function.imagick-compareimages.php

Biggest problem is that the documentation for that library is pretty much non-existing, so there will be a lot of trial-and-error involved. The PHP extension is a pretty thin wrapper around the ImageMagick library, so details of how the compareimages() function behaves can be found in the ImageMagick documentation.

edited Jul 28 '20 at 10:08

answered Jul 17 '10 at 09:42

kander

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if we're not depended upon a library.... how was that? – gumuruh Jul 23 '20 at 09:58
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Not sure I understand your question, gumuruh? If you don't want to depend on a library, you'd have to write the algorithms to parse + process image by yourself. ImageMagick is open source, so if you have way a lot of time (and are willing to sacrifice efficiency), I guess you could write a PHP-native implementation. – kander Jul 28 '20 at 10:05

score 18 · Answer 2 · answered Feb 06 '12 at 11:15

18

$md5image1 = md5(file_get_contents($image1));
$md5image2 = md5(file_get_contents($image2));
if ($md5image1 == $md5image2) {

}

answered Feb 06 '12 at 11:15

Hamada Mido

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Why not just use md5_file()? – Jason Nov 12 '12 at 01:37
14

That's really a file compare. Not so much an image compare which is what the question is. – Michael M Sep 20 '13 at 20:45
@Jason why not just ($image1 == $image2) – Peter Dec 16 '13 at 14:30
It's shown to be faster for me. Also seems to use less memory. – Jason Dec 16 '13 at 19:28
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I dont know why, but when comparing 2 identical images for me, md5 is failing and md5_file is working – Pablo Oct 31 '16 at 12:40
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If you are comparing images produced by different software (or even different libpng versions) then the same image can have different binary representations on disk, so MD5 comparisons will fail. – AnotherHowie Dec 31 '17 at 14:56
SIMILAR not THE SAME (ofc: md5 could be the same even if files ale completely different) – PawelN Nov 03 '18 at 10:17
what...? so what's the last solution? – gumuruh Jul 23 '20 at 09:58

score 11 · Answer 3 · answered Dec 18 '11 at 15:05

11

libpuzzle is a PHP extension that can compare images.

answered Dec 18 '11 at 15:05

pfrenssen

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score 4 · Answer 4 · edited May 23 '17 at 11:46

4

A similar question was asked on this Stackoverflow thread and I had developed something for my own use. Posting it here so that it may help others.

It takes two (or more images) and gives you options about checking the difference between them. Options like resolution to use, and strictness.

I wrote a more comprehensive blogpost on it as well.

edited May 23 '17 at 11:46

Community

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answered Nov 15 '13 at 17:13

Pat Cullen

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That is one awesome script actually. – Kalle H. Väravas Oct 06 '14 at 19:14

Leopoldo Sanczyk · Answer 5 · 2020-09-05T01:43:03.913

I coded a kind of Mean Square Error comparing method, using GD Graphics Library already included in PHP.

I was concerned about time, so I included some options such as:

Scaling the images before comparing.
Using only one RGB channel instead of perceived luminance of the image.
Calc a linear error difference instead of MSE.

But seeing the results, if you are doing bulk operations, perhaps the best is to hardcode the options to avoid the overhead of the conditional lines.

This is the code, with some test and a benchmark:

<?php
define('FILENAME1','./image1.png'); 
define('FILENAME2','./image2.png');    
define('OUTEXT','png'); //Output extension
    
/**
 * Calculate an arbitrary metric of difference between images
 * 
 * @param resource(gd) $img1 First image
 * @param resource(gd) $img2 Second image
 * @param int $resX Scaled width resolution
 * @param int $resY Scaled height resolution
 * @param int $pow Mean square error if 2, but could be another power
 * @param string $channel RGB channel or 'all' for perceived luminance
 *
 * @return float the calculated metric
 */
function diff($img1,$img2,$resX=false,$resY=false,$pow='2',$channel='all'){ 
    //Scaling to image 1 size for default
    if(!$resX||!$resY){     
        $resX=imagesx($img1); //width
        $resY=imagesy($img2); //height
    }
    //Use imagescale() function to scale the images 
    $thumb1=imagescale($img1,$resX,$resY);
    $thumb2=imagescale($img2,$resX,$resY);
    //Sum of errors
    $sum=0;
    for($x=0;$x<$resX;++$x){
        for($y=0;$y<$resY;++$y){
            //Read pixel bytes
            $bytes1=imagecolorat($thumb1,$x,$y);
            $bytes2=imagecolorat($thumb2,$x,$y);
            //Split the channel values from bytes
            $colors1=imagecolorsforindex($thumb1,$bytes1);
            $colors2=imagecolorsforindex($thumb2,$bytes2);
            //Choose image channel
            if($channel=='all'){
                //Perceived luminance
                $value1=sqrt(0.2126*$colors1['red']**2+0.7152*$colors1['green']**2+ 0.0722*$colors1['blue']**2);
                $value2=sqrt(0.2126*$colors2['red']**2+0.7152*$colors2['green']**2+ 0.0722*$colors2['blue']**2);
            }else{
                //RGB channel
                $value1=$colors1[$channel];
                $value2=$colors2[$channel];
            }
            $sum+=abs($value1-$value2)**$pow;           
        }
    }
    //Return mean of the error sum
    return $sum/($resX*$resY);
}

//Show image in HTML
function imgdraw($imgobj){      
    //Choose function
    $image="image".OUTEXT;
    //Capture the image data stream     
    ob_start();     
    $image($imgobj);        
    $data = ob_get_clean();     
    //Create and HTML img
    echo '<img src="data:image/png;base64,'.base64_encode($data).'" />';
}

//Load an image
function loadimg($filename){
    //Get filename extension
    $ext=substr($filename,strrpos($filename,'.')+1);
    //Create image object
    $imagecreate="imagecreatefrom$ext";
    return $imagecreate($filename);
}

//Test 
$img1=loadimg(FILENAME1);
$img2=loadimg(FILENAME2);
if( !$img1 || !$img2){
    //Problem reading the files
    die("Error loading image files");
}else{
    imgdraw($img1);
    imgdraw($img2);
}

//Times for 133x144 pixels png images
echo "<p>original size MSE perceived luminance: ".diff($img1,$img2)."</p>";
//time: 0.2281 seconds.
echo "<p>original size MSE green channel: ".diff($img1,$img2,false,false,2,'green')."</p>";
//time: 0.1364 seconds.
echo "<p>original size linear perceived luminance: ".diff($img1,$img2,false,false,1)."</p>";
//time: 0.1920 seconds.
echo "<p>original size linear green channel: ".diff($img1,$img2,false,false,1,'green')."</p>";
//time: 0.1351 seconds.
echo "<p>64x64 MSE perceived luminance: ".diff($img1,$img2,64,64)."</p>";
//time: 0.0431 seconds.
echo "<p>64x64 MSE green channel: ".diff($img1,$img2,64,64,2,'green')."</p>";
//time: 0.0293 seconds.
echo "<p>64x64 linear perceived luminance: ".diff($img1,$img2,64,64,1)."</p>";
//time: 0.0423 seconds.
echo "<p>64x64 linear green channel: ".diff($img1,$img2,64,64,1,'green')."</p>";
//time: 0.0293 seconds.
echo "<p>16x16 MSE perceived luminance: ".diff($img1,$img2,16,16)."</p>";
//time: 0.0028 seconds.
echo "<p>16x16 MSE green channel: ".diff($img1,$img2,16,16,2,'green')."</p>";
//time: 0.0027 seconds.
echo "<p>16x16 linear perceived luminance: ".diff($img1,$img2,16,16,1)."</p>";
//time: 0.0027 seconds.
echo "<p>16x16 linear green channel: ".diff($img1,$img2,16,16,1,'green')."</p>";
//time: 0.0018 seconds.
?>

score 3 · Answer 6 · answered Mar 24 '14 at 18:20

3

Image Comparison Function in PHP with GD Library http://www.robert-lerner.com/imagecompare.php

answered Mar 24 '14 at 18:20

TechNikh

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[link from wayback machine](https://web.archive.org/web/20140211060753/http://www.robert-lerner.com/imagecompare.php) – mfink Sep 28 '18 at 17:07

score 0 · Answer 7 · answered Aug 01 '12 at 06:22

0

If you are comparing just two files then hashing data and then comparing is perfect solution. If you are comparing large number of files, then better sort them first based on size and then compare only with same size.

answered Aug 01 '12 at 06:22

mrd081

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score 0 · Answer 8 · answered Jul 17 '10 at 09:36

0

Not sure whether it is that easy and a solution exits but probably you may get an idea of image detection from:

Face detection with PHP
Image Nudity Filter (Class)

answered Jul 17 '10 at 09:36

Sarfraz

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score 0 · Answer 9 · answered May 19 '23 at 11:32

If you are using php 8.1 or higher, you can use https://github.com/sapientpro/image-comparator library for that:

$comparator = new SapientPro\ImageComparator\ImageComparator();

$similarity = $imageComparator->compare('your-images/your-image1.jpg', 'your-images/your-image12.jpg');

echo $similarity; // 82.3

Compare 2 images in PHP

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